The present invention relates to a heat exchanger for cooling industrial liquids in general, and more particularly to an air-cooled heat exchanger for liquids which have a tendency to form deposits at least when the temperature thereof drops below a given level.
Air-cooled heat exchangers are already known and in widespread use. In industrial applications, that is, for cooling the above-mentioned industrial liquids, these heat exchangers include at least one bunch of cooling pipes which are provided with external ribs or fins thereon and past which cooling air is being advanced to withdraw heat from the cooling tubes and the fins thereof. More often than not, the cooling sections of these conventional heat exchangers have their externally ribbed cooling tubes arranged horizontally, parallel to one another, and in rows next to each other or in columns above each other. The liquid to be cooled is then introduced into the individual cooling sections and withdrawn therefrom after passing through the same.
A considerable problem exists when industrial liquids, which usually include particulate material and/or components which have a tendency to precipitate or deposit when the temperature of the liquid drops below a given level, are to be cooled, which resides in the fact that the interiors of the cooling sections may become clogged by the deposition and by the subsequent adherence of the particulate material or of the precipitated components, and that the cooling process is rendered ineffective or is interrupted altogether as a result of this.
This problem exists, for instance, but not exclusively, when tar-containing water obtained during the gasification of coal is to be cooled, which tar-containing water may, under certain circumstances, include considerable amounts of particulate materials in the form of coal dust or fly ash. Only when it can be assured that no particulate material will deposit at any region of the cooling system within the respective tubes, but that the particulate material will rather be kept in a suspension, is it possible to avoid the clogging of the heat exchanger. On the other hand, when the velocity of flow of the liquid to be cooled is too high, for instance, in an attempt to prevent the particulate material from depositing, there comes into existence another disadvantage that the particles which are entrained in and transported by the liquid to be cooled subject the sections of the cooling system to a substantial wear due to erosion, especially in the regions of the heat exchangers where the direction of flow of the liquid changes. Finally, this situation is further aggravated when the liquid to be cooled is tar-containing water by the fact that the temperature of the tar-containing water during the cooling thereof must not drop below a critical temperature, inasmuch as tar would precipitate from the water when this limiting temperature is reached and deposit within the cooling sections of the heat exchanger on the surfaces which bound the path of flow of the tar-containing water therethrough. This, in turn, brings about not only the danger that these tar deposits would reduce the flow-through cross-sectional area of the respective tubular sections possibly up to eventually full obstruction thereof, but also a reduction of the heat transmission or heat transportation as a result of the heat-insulating properties of these deposits so that the cooling effect of the heat exchanger will drop below that for which the heat exchanger has been designed. Even this latter problem is not exclusively encountered in the heat exchangers for cooling tar-containing water; rather, it occurs in a similar manner even in connection with cooling numerous other industrial liquids.
The drawbacks which have been discussed above with respect to the cooling of industrial liquids and which are detrimentally reflected in the cooling efficiency and cooling operation of the heat exchanger, are naturally the more important the larger the heat exchanger is to be made in order to be able to handle the volume of the liquid to be cooled, and, as a consequence thereof, the more complex the heat exchanger is to be constructed.
In order not to have to construct the heat exchanger, even with respect to the number of individual cooling branches provided therein for the liquid too complex and, possibly even more importantly, in order to be able to use cooling tubes having relatively large diameters of, for instance, 50 millimeters or more, it has already been proposed to cool the above-mentioned liquids by means of water. While it is true that the relatively large-diameter cooling tubes have the advantage that they are relatively easy to internally cleanse from time to time, an important disadvantage of this water-cooled heat-exchange system is that, precisely because of the cooling of the cooling tubes by cooling water, it is very difficult to cleanse the cooling tubes during the operation of the heat exchanger. For the latter reason, it is usually necessary in order to be able to cool the industrial liquid without interruption and to be still able to periodically internally cleanse the cooling tubes, to provide two separate heat exchangers in tandem so that the cooling tubes of one of these heat exchangers can be cleansed while the other heat exchanger cools the liquid flowing therethrough, and vice versa following the termination of the cleansing operation of the first-mentioned heat exchanger. On top of this, there is encountered the disadvantage that, for instance, when the liquid to be cooled is tar-containing water, high temperatures of the cooling tubes occur at the side of the cooling water because of the high entrance temperatures of the liquid to be cooled which are in the order of magnitude of approximately 170.degree. C., so that the danger of corrosion and of formation of deposits at the cooling water side of the cooling tubes is very pronounced. To avoid these consequences alone, very often the cooling tubes have to be made of relatively expensive alloyed steel and/or the cooling water must be circulated in a closed circuit for the cooling water to be re-cooled therein, in order to be able to perform the actual cooling operation with treated, fully desalted water.
The latter expedient would also be unavoidable under the circumstances where either the cooling water is not available in sufficient quantities or where the cooling water is too expensive because of the difficulties arising during the acquisition thereof.
Because the above-mentioned difficulties, the cooling even of industrial liquids by an air stream propelled by a blower presents itself as a desirable alternative. A particular advantage of this is that the cooling air is available in practically unlimited quantities anywhere inasmuch as it can be withdrawn from the ambient atmosphere and then blown against the cooling tubes. In view of the fact that the heat-transmission coefficient at the side of the liquid to be cooled is up to fifty times greater than on the side of the cooling air, it is mandatory under these circumstances to equip the heat exchanger with cooling tubes which are provided with external ribs or fins. When the cooling tubes are provided with the external ribs, the cooling surface which presents itself to the cooling air increases up to thirty times relative to the exposed surface of a simple circular cooling tube.
However, externally ribbed cooling tubes which are optimally usable and economical to manufacture are limited as to their inner diameter to a predetermined value which, in general, lies only between 25 and 38 millimeters. If the inner diameter of the cooling tubes were greater, the heat-transmission coefficients which determine the penetration of heat through the cooling tubes would be too low and thus the heat exchanger would be too expensive because of the increased cooling surface thereof. In this connection, it is to be considered that extremely high amounts of the liquid to be cooled, for instance, of tar-containing water, are encountered in industrial plants, such as those for coal gasification. This renders it often necessary to arrange up to three thousand cooling tubes or more which have lengths up to 12 meters in a cooling unit of the air-cooled heat-exchanger type.
Experience has shown that air-cooled heat exchangers of these dimensions cannot be so arranged, based on the current state of the art, that a faultless cooling operation can be assured under all conditions and while satisfying all of the above-enumerated requirements.
Namely, this would presuppose that the liquid to be cooled have always the same, sufficiently high, but not too high, flow velocity at all regions of the exchanger in order to avoid the possibility of deposition of and clogging by particles which are entrained in the liquid being cooled. In addition thereof, it would be required to so select the flow velocity of the liquid as to avoid or at least reduce possible erosions, especially at those regions of the heat exchanger where the direction of flow of the liquid being cooled changes. Moreover, it would be necessary to assure during the cooling operation that the liquid being cooled does not suffer a reduction in its temperature below the given critical limiting temperature at any region of the heat exchanger, in order to avoid the precipitation from the solution of such components which tend to precipitate at or below the limiting temperature, such as, for instance, tar out of tar-containing water. Finally, it would also be necessary, for instance, when the tar-containing water is the liquid to be cooled, to so coordinate the various temperatures which occur within the cooling system, that is, the inlet temperature of the liquid to be cooled which lies within rather narrow limits about, for instance, 170.degree. C., the exit temperature of the cooled liquid which amounts to, for instance, approximately 70.degree. C., the critical temperature which is, for instance, 60.degree. C. and at which tar would precipitate from the tar-containing water, as well as the temperature of the cooling air at the particular location of the heat exchanger which may be, for instance, at about 30.degree. C. in average and may drop, for instance, to as low as -5.degree. C., during the design of the heat exchanger, that an unproblematical cooling operation of the heat exchanger is obtained even when the conditions change, for example, when the temperature of the ambient air changes.
The satisfaction of all of these conditions is, for a variety of reasons, not possible or possible only after overcoming considerable difficulties, in the conventional air-cooled heat exchangers. In view of the fact that, in the conventional air-cooled heat exchangers of this type, the bunches of externally ribbed cooling tubes are so put together that the externally ribbed cooling tubes are welded or rolled at their ends in end plates which, at their sides which face away from the cooling tubes, are provided with distributing or collecting chambers which are either welded or detachably connected to the respective end plates, it is impossible for this reason alone to obtain a uniformly high flow velocity of the liquid throughout the system in installations using huge bundles of externally ribbed cooling tubes numbering up to fifty of such cooling tubes which are arranged next to one another and which are arranged at least partially in parallelism with one another with respect to the above-mentioned end chambers. This is attributable to the fact that, in dependence on the number of the cooling tubes which are arranged in parallel because of the configurations of the chambers or any partitions of the latter, the flow velocity of the liquid within the chambers drops considerably with respect to that obtained within the cooling tubes, which results, in time, in the formation of deposits and in clogging at least in the above-mentioned chambers.
A further disadvantage of the prior-art air-cooled heat exchangers for the purpose here under consideration resides in the fact that different temperatures of the liquid being cooled are obtained in the direction of the flow of the cooling air within the individual rows of the cooling tubes of the respective bundle of such tubes. Namely, in that row of the cooling tubes which is closest to the point of origin of the inflowing cooling air, a correspondingly higher cooling effect is obtained naturally. On the other hand, inasmuch as already pre-heated cooling air comes into contact with the cooling elements of the rows which are located downstream of the first-mentioned row in the direction of flow of the cooling air, these following rows of cooling tubes will be cooled less and less, the cooling effect being least pronounced at the row of the cooling elements which is arranged last as considered in the flow of direction of the cooling air. It follows from the above that a mixing takes place in the collecting chamber or chambers in which the externally ribbed cooling tubes open at their ends between the amounts of liquid which have been cooled to a greater or lesser degree, respectively. Thus, the temperature of the cooled liquid which can be measured at the outlet nipple of the bundle after the termination of the cooling process is an average temperature which results from the mixing of the branch streams of the cooled liquid which have respectively different temperatures.
Now, when the exit temperature of the cooling air is measured downstream of the bundle of the externally ribbed cooling tubes following the heat exchange therewith it will be established that the cooling air is heated up less with increasing cooling of the flowing liquid being cooled. This is attributable to the fact that, as the temperature differential between the cooling air and the liquid being cooled decreases in correspondence to the progress of the cooling process, less and less heat is carried out of the individual parts of the heat exchanger and, consequently, the cooling air which passes therethrough is also correspondingly heated up to a lesser extent. The danger that, for this reason, the liquid could be undercooled when the ambient temperature of the cooling air decreases, is particularly great in this situation. Therefore, the externally ribbed cooling tube bundles as they are usually used for the cooling of liquids, are not utilizable for the above-mentioned purposes wherein a local undercooling of the liquid is to be avoided under all circumstances.